The present invention relates generally to infrared (IR) imaging or staring arrays. More particularly, the invention concerns a micro-bolometric semiconductor IR staring array that provides substantially improved sensitivity in a bolometric device and structure exhibiting greatly increased manufacturability and thus high-performance yield.
High-performance infrared imaging applications require high sensitivity and TV quality in terms of clarity, contrast, resolution and continuity of flicker-free motion. Such applications include daytime and night-time personnel and vehicle pilotage, navigation and reconnaissance for military and commercial enterprises. Nevertheless, there traditionally have been two principal constraints against the development of such high-performance uncooled infrared imaging systems: substantial improvements are required in first, the detector and second, the readout circuitry of what will be referred to herein as an uncooled focal plane array (UFPA) that forms the eye of the system.
Typically, in the design and fabrication of infrared sensors utilizing bolometric detection mechanisms, it is appropriate to select an approach which provides the following features: 1) a large percentage of photon absorption (.gtoreq.90%) in the wavelength band of interest (typically 8-14 microns); 2) high thermal isolation of the detector element to maximize photon absorption-related temperature changes (typically 10.sup.-7 W/.degree.C.); 3) a high temperature coefficient of a measurable parameter (typically resistance at 2%/.degree.C.); 4) a thermal time constant to correspond to the electronic detector sampling rate (typically 10-20 msec); and 5) an electronic switch pixel address mechanism (typically a diode, bipolar or field-effect transistor (FET)) and interconnect circuitry within the pixel to access the bolometric detector element and to sample the infrared radiation-induced signal variation.
Conventional bolometric IR staring arrays have used temperature-sensitive resistors (thermistors) as the IR energy-sensitive or active detector elements thereof. Such resistors based on either metallic or semiconductor films have required suspension above or alongside the semiconductor array signal-sensing circuitry that takes the form of a diode or switchable transistor (an operating characteristic of which indicates to an array-scanning processor the amount of IR energy incident on the plural pixel array). Such thermal isolation structures and associated signal sensing circuitry are difficult to manufacture due to a large number of complex processes, and notoriously have produced yield problems in manufacture and attendant low-yield cost increases.
Briefly summarizing the invented staring array, the bolometric infrared detection element in each pixel within a two-dimensional array is a thermally isolated semiconductor device and associated photon absorption structure having a selectively forward-biased semiconductor junction, e.g. a selectively biased diode. Each pixel diode in the array serves as both a bolometric type detecting element and a switching element. Each diode in a given row of the IR pixel array to be sensed, or addressed, is driven at a constant voltage, rendering its bolometric response highly controllable in the forward-biased operating curve of the diodes in the addressed row. Diodes not being driven, due to their reverse bias, are in their off state, thus producing a small leakage current and thus make no significant contribution to the sensed current representing an addressed pixel's absorbed IR exposure. The row-addressed driven or active diodes, characterized by thermal time constants selected to be compatible with the staring sensor frame rate are sensed to produce a two-dimensional IR pixel image of a scene. This new bolometric pixel architecture simplifies the geometries as well as the cell structures while producing a large detector fill ratio of greater than approximately fifty percent. Significantly, manufacturing yields are greatly improved because of the topologic and process simplicities.
Accordingly, it is a principal object of the present invention to provide a simplification in the pixel structure that eliminates the need for a separate pixel-addressing device, incorporating this function into the bolometric semiconductor diode detection element.
It is a further object of the present invention to replace the conventional linearly resistive film bolometer with a forward biased semiconductor diode bolometer that provides increased temperature sensitivity.
Yet another object is to provide a bolometer characterized in that it provides inherent pixel addressing capability.
Still another object of the invention is to provide a cost-effective and high-yield array using processes and materials that are at least compatible, and perhaps synergistic, with standard integrated circuit (IC) manufacturing techniques and materials.
These and additional objects and advantages of the present invention will be more readily understood after a consideration of the drawings and the detailed description of the preferred embodiment.